The present invention is in the general field of perforated plastic film and especially relates to perforating of plastic film. The invention particularly relates to perforated plastic film, the metal screens or molding elements used in the vacuum perforation of plastic film and to a method of fabricating such screens.
Perforated plastic film has many useful applications. It is used in gardening and farming to prevent the growth of grass and weeds while permitting moisture to be transmitted through the film to the soil beneath. Perforated films have a multiplicity of regularly spaced apertures which allow permeation of liquid and air or other fluids. Such films can be used as a component of disposable garments for sanitary apparel purposes, such as napkins, diapers, or for hospital pads, bed or sleeping bag linings, and the like. In such composite structures, an exterior layer of film having the desired properties is provided which would be adjacent to the skin in a composite garment, and the garment would also include a filler layer or layers of absorbent fibrous material. An example of the use of perforated film for making disposable diapers is shown in U.S. Pat. No. 3,814,101.
A particular class of perforated film is described by Thompson, U.S. Pat. No. 3,929,135, issued Dec. 30, 1975. Thompson teaches an absorptive structure with a top layer of perforated film characterized by having a series of regular spaced small apertures in the form of tapered capillaries of certain dimensions ranges. In the finished article, these are directed inwardly to be in intimate contact with an absorbent fibrous material layer. The smooth side of the perforated film is thus in use in contact with the skin. Film as described by Thompson, in garment structure as outlined, maintains a dry and comfortable condition, even after transmission of fluids to the absorbent layer by the combined effects of the absorption and the resistance to back flow as a result of the relative length and surface properties of the tapered capillaries.
One of the earlier methods for vacuum perforation of plastic film is disclosed in U.S. Pat. No. 3,054,148. The patentee describes a stationary drum having a molding element or screen mounted around the outer surface of the drum and adapted to freely rotate thereon. A vacuum chamber is employed beneath the screen to create a pressure differential between the respective surfaces of the thermoplastic sheet to be perforated to cause the plasticized sheet to flow into openings provided in the screen and thereby cause a series of openings, holes or perforations to be formed in the plastic sheet or film.
One method for making film with tapered capillaries on one side thereof is shown in U.S. Pat. No. 3,054,148 issued Sep. 18, 1962, to Zimmerli. In this patent heated film is supported by a perforated screen and a vacuum applied to the underside of the perforated screen. Holes are pulled in the film in the direction of the vacuum beneath the screen thereby forming tapered capillaries in the film.
A variety of methods and apparatuses including particular types of perforating screens or rotatable molding elements have been developed over the years for particular perforation operations. Examples of these are U.S. Pat. Nos. 4,155,693; 4,252,516; 3,709,647; 4,151,240; 4,319,868 and 4,388,056. In U.S. Pat. No. 4,155,693, the screen is comprised of a series of perforated metal strips preferably welded together to form a cylinder. U.S. Pat. No. 4,252,516 provides a screen having a series of hexagonal depressions with elliptical holes centered therein. U.S. Pat. No. 3,709,647 provides for a rotating vacuum-forming roll having a circulating cooling medium therein.
U.S. Pat. No. 4,151,240 provides a means for cooling the film after it has been perforated and debossed. U.S. Pat. No. 4,319,868 sets forth an apparatus for making a thermoplastic film having raised bosses with perforated tips. A particularly constructed embossing roll for effecting the desired film pattern is disclosed. U.S. Pat. No. 4,388,056 discloses an apparatus for continuously forming an air-laid fibrous web having oppositely phased, cylindrically undulating side edges and a predetermined basis weight distribution. An air-laying drum has a honeycomb-type annular-shape frame including circumferentially extending ribs and transverse plates. A stationary adjustable air flow modulating means is disposed adjacent the radially inwardly disposed boundary of an arcuate portion of a circumferentially segmented annular-shape plenum, circumferentially spanning a plurality of plenum segments for adjusting a pressure drop across particular areas of the surface of the air-laying drum.
Vacuum perforation of thin plastic films involves the extrusion of molten polymeric materials such as polyethylene and other plastic polymers through a die. The hot melt web of film or plastic sheet exiting the die impinges on a rotating cylindrical screen which is mounted on a stationary vacuum drum or roll. The vacuum roll has an axial slot and a set of seals extending longitudinally along the length of its inside surface, beneath the area where the web of plastic impinges on the screen or molding element. A vacuum from inside the screen is directed through the slot in the vacuum roll. The vacuum present within the slot forms or molds the plastic film or sheet to the screen and perforates it through the holes of the screen. At the same time, an airflow is produced which cools the film.
An important component of the vacuum processing equipment is the cylindrical screen. This molding element defines aesthetic, tactile and mechanical properties of the film as well as the geometric pattern of the perforated film. In a preferred screen fabrication technique, the desired screen pattern is nickel plated on a specially prepared cylindrical mandrel. A seamless cylindrical nickel screen of any predetermined or desired pattern can be produced. Other metals, such as copper may also be used.
However, the prior art screens produce film with perforations that extend through the film at substantially a right angle to the surface of the film. Such perforations provide a direct line of sight and a direct path through the film. This feature of the prior art film is undesirable when the film is used in catamenial or incontinent applications as the collected fluid remain visible. Accordingly, there is a need for a perforated film having masking characteristics that reduce the visible presence of the collected fluids.
It is also desirable to have a film that does not provide a direct path through the film for fluids. Such a film can be utilized for protective clothing as fluids contacting the surface of the film will not have a direct path through the film. Such a characteristic greatly improves the protective quality of the clothing.